From: Daniel Vetter <daniel@ffwll.ch>
To: Pekka Paalanen <ppaalanen@gmail.com>
Cc: Daniel Vetter <daniel@ffwll.ch>, Simon Ser <contact@emersion.fr>,
Rob Clark <robdclark@chromium.org>,
Brian Norris <briannorris@chromium.org>,
Andrzej Hajda <andrzej.hajda@intel.com>,
David Airlie <airlied@linux.ie>,
Dmitry Torokhov <dmitry.torokhov@gmail.com>,
Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
dri-devel <dri-devel@lists.freedesktop.org>,
Doug Anderson <dianders@chromium.org>,
"open list:ARM/Rockchip SoC..."
<linux-rockchip@lists.infradead.org>,
"Kristian H . Kristensen" <hoegsberg@google.com>,
Thomas Zimmermann <tzimmermann@suse.de>,
linux-input@vger.kernel.org
Subject: Re: [PATCH v2 1/2] drm/input_helper: Add new input-handling helper
Date: Thu, 25 Nov 2021 16:30:27 +0100 [thread overview]
Message-ID: <YZ+sEw2ya80bYYaC@phenom.ffwll.local> (raw)
In-Reply-To: <20211122114342.0d23890f@eldfell>
On Mon, Nov 22, 2021 at 11:43:42AM +0200, Pekka Paalanen wrote:
> On Fri, 19 Nov 2021 17:11:07 +0100
> Daniel Vetter <daniel@ffwll.ch> wrote:
>
> > On Fri, Nov 19, 2021 at 04:04:28PM +0000, Simon Ser wrote:
> > > On Friday, November 19th, 2021 at 16:53, Daniel Vetter <daniel@ffwll.ch> wrote:
> > >
> > > > Random idea ... should we perhaps let userspace connect the boosting? I.e.
> > > > we do a bunch of standardized boost targets (render clocks, display sr
> > > > exit), and userspace can then connect it to whichever input device it
> > > > wants to?
> > >
> > > On IRC we discussed having user-space hand over a FD to the kernel. When the FD
> > > becomes readable, the kernel triggers the boost.
> > >
> > > This would let user-space use e.g. an input device, an eventfd, or an epoll FD
> > > with any combination of these as the boost signal.
> >
> > Can userspace filter eventfd appropriately like we do here? And can they
> > get at that maybe 2nd eventfd from logind or whatever there is on distros
> > where /dev access is locked down for compositors/users.
>
> (Mind, eventfd is a specific thing, see 'man eventfd', and evdev/input
> device fd is different.)
Yeah I was a bit sloppy, but I knew.
> I don't think any of that is any problem when userspace prepares an
> epoll fd to be given to the boosting machinery. The boosting machinery
> could have several different targets as well, PSR vs. GPU clocks and
> whatnot.
>
> I envision a compositor to maintain an epoll fd for boosting by
> adding/removing the same device fds to it that it already uses in its
> operations. I don't see any need to open new device fds just for
> boosting. It's only the epoll fd given to the kernel and after that the
> epoll set can still be changed, right?
>
> The boosting machinery would never actually read or write the
> registered fd(s), so it would not interfere with the normal operations.
> But it also means the fd will remain readable until userspace services
> it. Userspace may need to set up that epoll set very carefully to have
> it work right (e.g. edge-triggered?).
>
> If your input handling is in a different process than the DRM poking
> for some reason, the epoll fd should still work if:
> - it is possible to use SCM_RIGHTS to pass the epollfd from the
> input process to the DRM process, and
> - you cannot extract the watched fds from an epoll fd.
>
> Do we have those assumptions today?
>
> Then the attack surface in the DRM process is limited to changing the
> epoll set of which fds can trigger boosting, but the DRM process can do
> that anyway. I also presume the input process can still add and remove
> fds from the epoll set even afterwards.
>
> > I do agree that if we can do this generically maybe we should, but also
> > the use-case for input boosting is pretty well defined. I think it's just
> > about making sure that compositors is in control, and that we don't make
> > it worse (e.g. with the sr exit adding latency when the compositor can
> > redraw quickly enough).
>
> The epollfd design sounds very good to me. One can register an
> arbitrary set of fds with it, and use even eventfds in the set to have
> purely software triggers.
Yeah I think just allowing to internall poll on any arbitrary fd sounds
like a neat interface. Userspace should then be able to do whatever it
wants to.
-Daniel
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
next prev parent reply other threads:[~2021-11-25 15:39 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-11-17 22:48 [PATCH v2 0/2] drm: Support input-boosted panel self-refresh exit Brian Norris
2021-11-17 22:48 ` [PATCH v2 1/2] drm/input_helper: Add new input-handling helper Brian Norris
2021-11-18 9:05 ` Daniel Vetter
2021-11-18 19:30 ` Brian Norris
2021-11-19 10:01 ` Daniel Vetter
2021-11-19 19:07 ` Brian Norris
2021-11-25 15:25 ` Daniel Vetter
2021-11-18 10:39 ` Pekka Paalanen
2021-11-18 23:30 ` Rob Clark
2021-11-19 9:54 ` Pekka Paalanen
2021-11-19 15:53 ` Daniel Vetter
2021-11-19 16:04 ` Simon Ser
2021-11-19 16:11 ` Daniel Vetter
2021-11-22 9:43 ` Pekka Paalanen
2021-11-25 15:30 ` Daniel Vetter [this message]
2021-11-19 16:44 ` Rob Clark
2021-11-19 16:50 ` Doug Anderson
2021-11-19 1:46 ` Brian Norris
2021-11-19 10:38 ` Pekka Paalanen
2021-11-19 15:56 ` Daniel Vetter
2021-11-22 9:25 ` Pekka Paalanen
2021-11-30 20:35 ` Brian Norris
2021-12-07 3:16 ` Peter Hutterer
2021-11-17 22:48 ` [PATCH v2 2/2] drm/self_refresh: Disable self-refresh on input events Brian Norris
2021-11-18 9:11 ` Daniel Vetter
2021-11-18 8:34 ` [PATCH v2 0/2] drm: Support input-boosted panel self-refresh exit Simon Ser
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